单选题 The forest from which Man takes his timber is the tallest and most impressive plant community of Earth. In terms of Man's brief life it appears permanent and unchanging, save for the seasonal growth and fall of the leaves, but to the forester it represents the climax of a long succession of events.
No wooded landscape we see today has been forest for all time. Plants have minimum requirements of temperature and moisture and, in ages past, virtually every part of Earth's surface has at some time been either too dry or too cold for plants to survive. However, as soon as climatic conditions change in favour of plant life, a fascinating sequence of changes occurs, called a primary succession.
First to colonize the barren land are the lowly lichens, surviving on bare rock. Slowly, the acids produced by these organisms crack the rock surface, plant debris accumulates, and mosses establish a shallow root-hold. Ferns may follow and, with short grasses and shrubs, gradually form a covering of plant life. Roots probe even deeper into the developing soil and eventually large shrubs give way to the first trees. These grow rapidly, cutting off sunlight from the smaller plants, and soon establish complete domination—closing their ranks and forming a climax community which may endure for thousands of years.
Yet even this community is not everlasting. Fire may destroy it outright and settlers may cut it down to gain land for pasture or cultivation. If the land is then abandoned, a secondary succession will take over, developing much faster on the more hospitable soil. Shrubs and trees are among the early invaders, their seeds calmed by the wind, by birds and lodged in the coats of mammals.
For as long as it stands and thrives, the forest is a vast machine storing energy and the many elements essential for life.

单选题 Why does the forest strike mankind as permanent?
A. The trees are so tall.
B. It is renewed each season.
C. Our lives are short in comparison.
D. It is an essential part of our lives.

单选题 What has sometimes caused plants to die out in the past?
A. Interference from foresters
B. Variations in climate
C. The absence of wooded land
D. The introduction of new types of plants

单选题 In a "primary succession", what makes it possible for mosses to take root?
A. The type of rock
B. The amount of sunlight
C. The amount of moisture
D. The effect of lichens

单选题 What conditions are needed for shrubs to become established?
A. Ferns must take root.
B. The ground must be covered with grass.
C. More soil must accumulate.
D. Smaller plants must die out.

单选题 Why is a "secondary succession" quicker?
A. The ground is more suitable.
B. There is more space for new plants.
C. Birds and animals bring new seeds.
D. It is supported by the forest.